This generates an unsigned download URL for hello.txt. This works
because we made hello.txt public by setting the ACL above.
This then generates a signed download URL for secret_plans.txt that
will work for 1 hour. Signed download URLs will work for the time
period even if the object is private (when the time period is up, the
URL will stop working).

Note

The java library does not have a method for generating unsigned
URLs, so the example below just generates a signed URL.